1. Field of the Invention
The present invention relates to the preparation of diesters. More particularly, the invention relates to the preparation of 1,1-diesters by the direct hydrogenation of carboxylic acid anhydride in the presence of an insoluble metal hydrogenation catalyst and a strong protonic acid.
2. Description of the Prior Art
1,1-Diesters are useful intermediates and solvents. Ethylidene diacetate, for example, is a well-known compound classically prepared by the reaction of acetaldehyde and acetic anhydride. More recently, it has been prepared in homogeneous solution by the hydrogenation of acetic anhydride catalyzed by a soluble noble metal-biphyllic ligand complex (see U.S. Pat. No. 3,579,566). In this patented process, the noble metal catalyst is maintained in solution by a variety of biphyllic ligands, e.g., organic phosphines, arsines, or stilbenes, such as trimethyl phosphine, triphenyl phosphine, etc. Reaction conditions include temperatures of 50.degree. C. to 250.degree. C., pressures from 1 to 300 atmospheres, and from 0.002 to 2.0 weight percent noble metal catalyst. The examples show low conversions (66%) and yields (11%), even after reacting for 2 hours at 100.degree. C. plus 2 hours at 200.degree. C. under hydrogen pressures ranging from 800 to 1200 psi. The Rosenmund Reduction (reference: Organic Reactions, Vol. IV, J. Wiley and Sons, New York, 1948, p. 362) involves the hydrogenation of an acid chloride to an aldehyde over a suitable catalyst, usually palladium.